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Aviation History
1963
1963 - 1211.PDF
FLIGHT International, 11 July 1963 37 ARMCHAIR STOL Flying the Breguet 941 at Northolt AS reported in a preceding World News page, the Breguet 941 spent several days at Northolt last week. I went to see the aircraft and flew it for about 20min at 50kt with 85° of flap and one engine throttled for some of the time. From this and extensive discussion with Breguet test pilot Bernard Witt, I gleaned the following details. At the moment the outer sections of the double flaps are less sharply deflected and their rearward portions are used as ailerons. These are backed up by extensive spoilers and by the differential control of the pitch of the outer propellers applied during both aileron and rudder movement. It is now intended to drop the normal aileron action and deflect the outer flaps to the same angle as the inners, thus achieving yet more drag. Spoiler and propeller pitch-change will then be used in all flight conditions for lateral control. Pitch attitude control is by normal elevator attached to an inverted-aerofoil tailplane, whose incidence can be varied over a very wide range from a "rocking-platform" switch on the control column. Though the tailplane is at present independent, it will later be interconnected with flap extension to compensate auto matically for the trim changes caused by the lifting range of flap travel. Up to 75° of flap provides mainly lift, and increasing drag results from further extension. The present rudder is a two-section unit, the inner (forward) of which is manually locked by switch at speeds over lOOkt. At lower speeds the outer rudder moves at twice the amplitude of the inner unit. In the production aircraft the rudder will be horizon tally split into upper and lower halves, after the manner of the VC10, each half having two hydraulic boosters, and the progressive disabling of the lower portion, in proportion to increased airspeed, will be controlled by a q feel unit. All aerodynamic controls are fully powered with, for the moment, a purely artificial spring-feel box under the control column, which is a short "broken" stick with a standard French pistol grip. The q feel unit will be added to give an approximately constant stick force per g through the flight envelope; and a yaw damper may also be fitted. But it is remarkable that handling characteristics without these aids have been shown to correspond closely to the AGARD standards, and where these are not met, the handling is not uncom fortable. Aileron forces at low speeds are at the moment quite light and a very considerable rate of roll can be developed. I per sonally like such light controls, as do the French, but the NASA pilots suggested heavier forces and lower response. The four free-turbine Turmos are controlled by conventional Breguet 94/ flight deck with angle of attack lights above the coaming, four throttles and transmission governor lever on the console and co pilot's combined power lever beside them. Note the pistol-grip control CO/umn "Flight International" photograph throttle levers and the transmission shaft has its own speed-gover nor lever, as well as a brake to stop the propellers during ground running. Any or all of the throttles can be linked to the main power lever by which the pilot normally controls engine power in flight. The flexible Hispano Suiza cross-shaft runs unloaded when all four engines deliver equal power in normal flight, and propeller pitch is automatically controlled by an "hydraulic central" mixing box. Power reduction or stopping of one engine is undetectable in flight except by the reduction in overall power, where this actually affects performance. Reverse pitch on all four propellers is achieved by disengaging a mechanical lock on the power lever handle with the little finger, pulling the handle out and up over a mechanical stop, moving it down through two mechanical stops which disengage the propeller fine-pitch stops and reverse actuator cylinders, moving a switch with the thumb to arm the reversing electrics and then pressing the handle fully down. The propellers reverse immediately and full power can be applied with normal forward power-lever movement in H to 2sec, without first throttling back. The last electric safety is now superfluous and will be removed. Setting of the outer propellers at zero thrust for the approach has not been tried in this aircraft and would in any case only give a very small reduction in approach speed. The Hispano Suiza shaft is performing very well and the company hope in due course to clear it for 2,000hr flight without even lubrication. In addition, the flaps can be "beeped" up or down by a rocking switch on the power lever. The pilot therefore sets up the approach angle of attack according to a naval three-light indicator (green correct, yellow too low, red too high), sets power appropriate to the conditions and "beeps" the flaps to adjust the rate of descent. The angle of attack indicator is above the coaming and is the only instrument to watch. The hands need not leave their handgrips throughout the approach. Normal short landings are made without using wheel-brakes, but they can be applied by toe pedals to achieve ultimate retardation. Nosewheel steering is used for taxying. The undercarriage is designed normally to absorb touch-down descent of 13ft/sec, but has been subjected to 15.1ft/sec with only minor damage following inadvertent full-power propeller reversal with the aircraft about five feet off the ground. The Jockey main gear can be tilted to alter the fuselage ground angle for loading. I flew the 941 briefly in the circuit at Northolt and found that it felt like a fairly large aircraft flying very slowly—the same kind of inertia—until I appreciated the astonishingly low airspeed of 50kt. We had about a dozen passengers and four hours' fuel on board. The more I turned left and right the more I realized that the 941 was very manoeuvrable and light, with slight spring feel in the ailerons and moderate spring resistance in the elevator. The rudder was very light and had to be used a fair amount to hold the nose straight while rolling and damp out occasional yawing, but the aircraft remained virtually undisturbed by changes of about 15kt in speed, alterations of about 20 per cent in power and reduction of flap angle from 85° to 75° and back—this last really surprising in view of the tremendous deflections involved. Witt cut one engine and relit it without any traceable effect on control. Elevator trim moved imperceptibly, but could be used to set hands-off flight level or in a turn. Lowering of the undercarriage was also undetectable on the controls. The 941 was almost as easy to fly as a light aircraft. Speed remained remarkably constant during manoeuvres. NASA say that 20 to 25 landings would serve to train a pilot to achieve the full STOL performance; Breguet claim that 10 or 12 would suffice. The angle of attack lights are green at the normal approach angle of 3°, and red appears at 10°. A stick shaker beings at 14° and buffeting at the tail begins at about 25°. The aircraft has been taken to 35° with and without power, without encountering a stall, but more than this is not advisable. The Ryan VZ-3 was taken to 30° and then lost tailplane effectiveness, flipped over backwards and the pilot had to eject downwards. As nowhere near these angles of attack are used in landing, this is not really germane to normal operation. The aircraft is only half flared from its 3° angle of attack on approach and less than full elevator is required to do so. In fact the remarkable factor in flying this rather exotic aircraft is that, at its minimum speeds, handling, angle of attack and engine control are absolutely normal from the pilot's point of view, with out autostabilization, specialized feel systems or automatic inter connections. This compares extremely favourably with present operational short-landing aeroplanes, while also giving far superior performance. c. M. L.
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